1. Field of the Invention
This invention relates to a method and apparatus for simulating a sonar device and more particularly to an apparatus and method for simulating a multipath sonar system.
2. Description of the Prior Art
Active and passive sonar systems are well known. An active sonar system generally comprises a transmitter and acoustic transducer that produces a scattering and propagating wave front. A receiving transducer array detects the incidence of a reflected portion of the wave front. Passive sonar systems consist substantially of the receiving transducer array, as the detected object becomes the transmitter either by means of noise from the object or operation of active sonar devices.
Sonar operators must necessarily become familiar with the devices they operate. Part of this training usually includes large amounts of time practicing with simulation devices. Prior simulation devices have generally operated by using wave interpolation models (e.g., models operating according to the spherical spreading law, to ray tracing, or to a wave model) at positions and frequencies of interest or by interpolating the acoustic pressure at the transducer face.
For example U.S. Pat. No. 3,789,128 to Murphree discloses an active signal generator, a passive signal generator and an echo generator to provide simulated multipath sonar signals. The active signal generator generates a first signal that is divided into two portions. One portion of the signal is delayed and phase shifted and then recombined with another unshifted portion to produce an input signal to an echo synthesizer. The passive generator produces a second signal that likewise is divided into portions with one portion delayed and phase shifted and then combined with the other portion to produce a combined signal. The output of the echo synthesizer and the combined signal from the passive signal generator are then combined to provide signals intended to simulate the characteristics of a multipath sonar signal.
U.S. Pat. No. 4,446,542 to Beckerle discloses an acoustic ocean measuring system that includes several sound pulse emitting floats in an area at determinable positions and several hydrophones for receiving the emitted signals. The system operates by receiving the emitted pulse signals to determine, over a fixed period, peak intensities of the signals received at the hydrophones and then comparing the peak intensities with a predicted peak intensity to indicate anomalies in the area.
U.S. Pat. No. 4,736,199 to Chadwick et al. discloses apparatus for receiving an incident acoustic wave along an array of hydrophones. The received signals are delayed and processed to provide an output representative of the frequency spectrum of the incident wave for various angles of incidence. The processed signals are then accumulated or stacked over time to produce lofargrams for display.
U.S. Pat. No. 5,150,336 to Sullivan et al. discloses a frequency dispersive transmitting array for propagating composite wave energy that emulates narrow-band beams of different frequencies radiated in directions relative to an array of linearly oriented radiating elements. The array of radiating elements is driven by a multi-frequency signal that is time delayed relative to each radiating element. The time delay corresponds to a multiple of the dominant frequency of the period of the signal radiated in a direction broadside to the array. The processing of the emission signal also includes window weighting circuitry to aid in the beam forming of the emitted signals from each of the radiation elements. The frequency of the emitted signal at a target or reflected from the target indicates the bearing angle from the radiation emitters to the target.
Other United States Letters Patent disclose various prior art apparatus or methods for simulating reverberation in multipath sonar systems or overcoming reverberation in multipath environments. For example U.S. Pat. No. 3,713,081 to Murphree discloses a sonar simulator that simulates sonar echoes with Doppler effects characterized by frequency shifts proportional to the transmitted frequencies. The simulator includes apparatus for storing digital representations of sonar signals at a predetermined rate. The apparatus then reads out the signals at a rate determined by a simulated rate of change in the range between the simulated target and receiver. The simulator further includes apparatus for attenuating and echo highlighting the simulated echoes depending on the simulated orientation angles between the target and the receiver.
U.S. Pat. No. 3,829,596 to Murphree discloses an electrical system for simulating sonar reverberation from sonar signals stored in memory. The stored signals replicate sonar signals at differing times corresponding to receipt of scatters from various ranges. A reference amplitude signal generator provides reference signals to adjust the amplitude and number of the replica signals for better representation. The apparatus also includes number generators, logic and comparators to simulate signals along various azimuths for simulation of multi-beam sonar receivers.
U.S. Pat. No. 4,393,483 to Hammond et al. discloses an apparatus for evaluating the performance of a sonar buoy system that includes a generator for generating simulated target information. The simulation section comprises a reverberation simulator, a signal synthesizer, bearing, echo level and sea state controllers, three noise sources, multipliers and summer circuitry. The synthesizer generates seven frequencies with adjustable phase. The generated signals provide bearing and echo characteristics by controlling the amplitude ratios and phase differences between the signals by multiplying the signals by constants. The noise sources generate noise signals that are manipulated and signed with the amplitude and phase multiplied signals to provide a composite signal. Combining the composite signal with the processed generated signals provides a simulated multipath sonar signal. U.S. Pat. No. 4,972,379 to Harris, Jr. discloses apparatus for simulating reflected sonar signals from under water targets having a transmitter which broadcasts sonar signals through a given current of water toward a receiver. The simulator includes first and second sawtooth wave generators with the frequency of the second output greater than the first output. The output of the first generator is amplitude, voltage, and duration adjustable according to a predetermined simulated target. The output of the second generator and the adjusted outputs of the first generator are combined to produce a composite signal representing a preselected target length, aspect and range and a second composite signal representing target aspect, length, range and shadow lengths. The second composite signal is converted to a third composite signal containing target range, length, a shadow length and highlights. The output of the second generator is also selectively gated and used to generated a simulated reverberation signal corresponding to the gating rate. This reverberation signal and the second composite signal are combined to produce a fourth composite signal that in turn is combined with the second composite signal to produce a fifth composite signal that represents a simulated multipath sonar signal.
U.S. Pat. No. 5,301,167 to Proakis et al. discloses an underwater acoustic communication system that enables high data transfer rates by use of feed-forward and feed-back equalizers, sample clocking, and doppler compensation. The system uses coherent phase modulation and demodulation of the received signals to overcome the reverberation and other multipath characteristics of underwater transmissions.
Other prior art apparatus can produce lofargrams in a multipath sonar simulator. For example the previously cited Chadwick et al U.S. Pat. No. (4,736,199) discloses apparatus for receiving and processing hydrophonic signals. The circuitry accumulates the signals in data blocks that represent lofargrams.
U.S. Pat. No. 4,964,408 to Chittineni discloses a process for tomographic development of seismic section data produced by a seismic source and receivers having known positions. Forward modeling provides estimated time travel intervals for the seismic waves travelling from the source to the receivers. The area between the source and receiver are divided into two dimensional cells with associated velocity coefficients. Ray tracing utilizing the estimated travel time, position orientation used to derive a travel parameter estimation of seismic travel time.
Still other prior art apparatus can produce autocorrelation coefficients. U.S. Pat. No. 3,676,565 to Rowe discloses a method of synthesizing time domain wave forms from frequency domain data with improved resolution in middle and lower frequencies for use as a sonar simulator. The disclosed method separates sensed frequency signals into predetermined frequency bins. The frequency bins are then Fourier transformed by using inverse or fast Fourier transforms to obtain time domain data. Adding the time domain data of each of the groups in corresponding times provides a composite time domain wave form.
U.S. Pat. No. 4,562,312 to Duttweiler discloses a method for estimating the delay in a communication circuit by subsampling incoming and outgoing signals to obtain a cross-correlation between the incoming and the outgoing signals to adjust a delay circuit to cancel echoes in the communication circuit.
U.S. Pat. No. 4,920,521 to Yoshie discloses apparatus for interpolating delay periods in transmitted ultrasonic signals and producing therefrom delay time data supplied to delay circuits. A predetermined delay time added to the interpolated delay time modifies the timing of transmission of ultrasonic signals and is also used in delaying ones of received signals being passed to summing circuits for summation with other received signals.
Yet other apparatus can produce a cross-correlation of received signals. As one example, U.S. Pat. No. 3,872,293 to Green discloses apparatus for performing time and space Fourier transform processing of optical signals from a sonar array. The process includes modulating coherent light with signals from an array of acoustic signals generated by hydrophones. After Fourier transform processing of the generated signals, transformed signals are mapped to produce an optical space model of the acoustic array.
U.S. Pat. No. 4,106,103 to Perreault discloses apparatus employing an automatic equalizer to calculate an equalization transfer function for received signals and perform the equalization within a frequency domain. Overlapping moving window samples are employed together with discrete Fourier transforms and inverse discrete Fourier transforms to provide an equalized time domain output signal.
U.S. Pat. No. 4,208,732 to Ruehle discloses apparatus for receiving seismic information signals from spaced receivers that are successively time shifted to provide a plurality of reference traces. The apparatus cross-correlates the reference traces with the individual seismic information signals to produce a correlation function to enhance the primary reflection signals within the seismic information signals.
U.S. Pat. No. 4,231,103 to Timm discloses a spectral analysis system with a fast Fourier transform processor that receives a time domain signal from a sonar receiver. The spectral analysis system estimates the signal strengths at discrete intervals for a frequency domain. Each of the time estimated signal strengths equals the frequency domain coefficients of the fast Fourier transform of the time domain signal generated when the time domain signal is sampled at a number of intervals equal to the number of initial estimates. A spectral recorder receives the adjusted estimates and records the frequency components of the time domain signal and relative strengths thereof.
U.S. Pat. No. 4,349,915 to Costas discloses apparatus for reproducing transmitted information signals that are subject to multipath delay spread effects and doppler frequency shift effects. The apparatus sequentially shifts the nominal frequencies of successively received pulses through a plurality of frequencies to determine the frequencies to which the transmitted pulses are doppler shifted and the multipath delays of each pulse. The frequency and delay information enables the reproduction of the actual transmitted information signals from the arrival pulses.
U.S. Pat. No. 4,562,312 to Duttweiler is previously described. It uses a delay circuit to cancel echo signals in the received signal.
These methods for simulation of sonar receivers suffer from various drawbacks. For example, those simulators that rely upon the spherical spreading laws do not and cannot mimic or simulate the acoustic phenomena, such as focusing and shadows, associated in actual multipath sonic environments. Simulators based upon ray modeling tend to be efficient but do not produce acceptable results in the demarcation of areas, often called caustics, that separate shadows and focussed regions in the simulated environment. To counter this limitation ray model simulators use factors that attempt to correct for the sounds transmitted in the caustic region. However, such prior art corrections have been highly inefficient in simulations when lower frequency signals are involved and when the simulated environment varies in range and azimuth.
Prior art simulators that rely on generation of wave models are generally inefficient at frequencies corresponding to sonar systems. They fail to adequately simulate multipath data and to enable simulation of reverberation. Additionally, wave model simulators for simulating passive sonar systems fail to reproduce the acoustic pressures of hundreds of frequencies with sufficient accuracy and efficiency. Large numbers of frequencies are necessary for producing sufficient simulated data to produce autocorrelation, lofargram and cross-correlation analysis necessary to simulate a passive sonar receiver. Due to the rapid fluctuation usually associated with actual sonar systems, prior art attempts to interpolate the acoustic pressures over various frequencies have proven to be unrealistic and inefficient.
Thus, the prior art fails to provide a method and apparatus that simulates in a relatively accurate manner a multipath sonar device, including, for example, enabling simulation of reverberation, lofargrams, autocorrelation coefficients and cross-correlation coefficients in a multipath sonar system. Some of the prior art devices even require actual sound emitting devices and hydrophone arrays. This defeats the purpose of seeking a relatively low cost and convenient multipath sonar system simulator.